and the whole "siege army" appealed to me for Iron Warriors
I'm kinda partial to siege armies as well. I'd buy a Death Korps army (the Imperial Guard's seige army) if they weren't so insanely expensive. They come from Forgeworld so their models cost at least double what models from Games Workshop proper cost.
Warhammer 40k would make a good setting for an RPG, either tabletop or PC. It's got decades of developed lore and artwork; it has myriad factions and sub-factions that can be adversaries, competitors, or allies of convenience; various types of locations; a wide variety of equipment for players to use; and it's built for nonstop violence, which is really the only type of character interaction that RPG design has truly nailed, imo, so the game design wouldn't need to be innovative, just solid, and the basic narrative would be easy for players to grasp.
I also wouldn't mind seeing a 40k total conversion mod for X-COM 2. I liked Dawn of War, but for some reason DoW2 didn't grab me like X-COM 2 did. I'm not sure why. I never played DoW3.
Well you're in luck because both of those exist on PC. For an RPG, you might want to look into Inquisitor: Martyr. It's a Diablo 3-esque action RPG where you play as an Inquisitor.
For a game that plays like XCOM, there is Mechanicus. I haven't played it yet, but it has very good reviews on Steam. You play as an Adeptus Mechanicus expedition raiding a Necron tombworld to steal their shiny technology. From what I've seen it plays a lot like XCOM.